[Base Excess] vs [Strong ION Difference]

Abstract

Blood [base excess] ([BE]) is defined as the change in [strong acid] or [strong base] needed to restore pH to normal at normal PCO₂. Some believe that [BE] is unhelpful because [BE] may be elevated with a “normal” [strong ion difference] ([SID]), where a strong ion is one that is always dissociated in physiological solution, and where [SID] = [strong cations] — [strong anions]. Using a computer simulation, the hypothesis was tested that [SID] = [SID Excess] ([SIDEx]), where [SIDEx] is the change in [SID] needed to restore pH to normal at normal PCO₂. The most current version of the plasma [SID] ([SID]_p) equation was used as a template, and an [SIDEx] formula, of the Siggaard-Andersen form, derived: ${\left[ {{\text{SIDEx}}} \right]_p} = {\left[ {HC{O_3}^ - } \right]_P} - 24.72 + \left( {p{H_p} - 7.4} \right) \times \left( {1.159 \times {{\left[ {{\text{alb}}} \right]}_p} \times + 0.423 \times {{\left[ {Pi} \right]}_p}} \right)$. [SID] was compared to [SIDEx] over the physiologic range of plasma buffering, and it was found that [SIDEx] varied by ~ 15 mM at any given [SID], thereby faulting the hypothesis. It is concluded that [SID] can be “normal” with an elevated [SIDEx], the latter being an expression of the [BE] concept, and a more helpful quantity in physiology.

The “metabolic” componenet of a given acid-base disturbances is usually estimated as whole blood [base excess] ([BE]_WB), where [BE]_WB is defined as the change in [strong acid] or [strong base] need to restore plasma (pH_p) to 7.4 at PCO₂ of 40 Torr^1–3. However. the [BE] approach has been criticized as “Inadequate for interpretation of complex acid-base derangements such as those seen in critically ill patient^4–5.” The proposed alternative is the strong ion difference (SID) method, where a strong ion is one that is always dissociated in solution, and where [SID] = [strong cations - [strong anions]^4–8.

On the one hand, it does not seem possible, by the definitions of these entities, to change [SID] without also changing [BE]. On the other hand, a selected group of critically ill patients with hypoproteinemia has been reported in whom [SID] was “normal” (i.e. ∼ 40mEq1¹) but [BE]_WB clearly increased^4,5,9. The idea was that hypoproteinemia caused the alkalosis, due to a deficiency of plasma weak acid buffer, necessitating increased [HCO₃]_p to maintain electrical neutrality. How could [SID] be “normal”, but [BE] increased? The purpose of the current exercise was to address this question. An [SID excess] ([SIDex]) formula was developed, conceptually identical to Siggaard-Andersen’s [BE], and [SID] was compared to [SIDex] over the physiological range of plasma [albumin] ([alb]_p), plasma [phosphate] ([Pi]_p), and plasma pH (pH_p).